In an effort to assure the stable location of an endocardial pacing electrode, a variety of approaches have been employed. One approach in common use today is to employ an endocardial pacing lead that has a sharpened tip, active fixation helix located at its distal end. The active fixation helix may either be a pacing electrode or may be located adjacent the pacing electrode(s) of unipolar, bipolar and multi-polar leads. Typically, the active fixation helix is rotated by some means from the proximal end of the lead and screwed through the endocardium into the myocardium to permanently fix the electrode in operative relation to cardiac tissue. A number of different varieties of endocardial screw-in pacing and cardioversion/defibrillation leads have been clinically used or proposed for use over the years.
Current endocardial screw-in leads have evolved from early epicardial and endocardial screw-in lead designs that employed wire coils or fixation helixes that were fixedly mounted to extend distally from the lead body distal end in axial alignment with the lead body axis. Epicardial screw-in leads were proposed for affixation of the sharpened fixation helix into the myocardium at a site exposed through a surgical exposure prior to endocardial screw-in leads. Such epicardial screw-in leads are exemplified by those disclosed in U.S. Pat. Nos. 3,472,234 to Tachick et al. and 4,000,745 to Goldberg. Later, endocardial screw-in leads, exemplified by those disclosed in U.S. Pat. No. 4,146,036 to Dutcher et al. and in German Patent No. 2533766, 2539553, issued to Osypka, were developed and clinically used. In these early epicardial and endocardial screw-in leads, rotation of the fixation helix into the heart tissue is accomplished either by rotation of the entire lead, or by rotation of a stylet with a screwdriver tip that engages a slot located internal to the pacing lead distal end or the fixation helix itself.
The exposed sharp tip of the fixation helix makes it difficult to advance the fixation helix transvenously from a skin incision into the right atrium and through the tricuspid valve or the coronary sinus ostium if the electrode(s) is to be fixed in the right ventricle or the coronary sinus or branching coronary vessel. Thus, many endocardial screw-in lead designs have been disclosed that employ fixation helixes that housed within a housing or protective sheath at the lead body distal end during advancement of the pacing lead through the venous system and the tricuspid valve without the possible danger of tissue damage caused by the extended fixation helix distal tip. A mechanism is provided to then extend the fixation helix distally from the housing or sheath so that it may be screwed into the heart tissue. An early version of such a retractable screw-in lead is illustrated in U.S. Pat. No. 4,106,512 issued to Bisping. In this example, the fixation helix is attached to the distal end of the coiled wire conductor extending proximally to the lead connector assembly and functions as an electrode. The fixation helix is advanced out of the housing the lead body distal end by rotation of the coiled wire conductor within a lead body lumen.
An alternative approach is illustrated in U.S. Pat. No. 4,217,913, issued to Dutcher wherein the fixation helix is mounted for rotation out of the distal housing. A screwdriver tip stylet is extended through a stylet lumen into engagement with a shaped receptacle of the fixation helix mounting and rotated at the stylet proximal end to rotate the fixation helix out of the lead body distal end. In this case, the fixation helix serves only to attach the lead body distal end to heart tissue, and a separate ring electrode(s) is mounted to the lead body and coupled to the lead conductor(s) for pacing and sensing.
Yet another approach is illustrated in U.S. Pat. No. 4,570,642 issued to Kane et al. wherein the fixation helix is located on a member that is slidable within a receptacle at the lead body distal end. The fixation helix is advanced from a lead body distal end housing by insertion of a cylindrical stylet through the stylet lumen into engagement with the slidable member to push the fixation helix distally and out of the housing. The fixation helix is then screwed into the tissue by rotation of the entire lead from the lead body proximal end.
The above-cited patents illustrate alternative approaches for attaching a fixation helix to cardiac tissue within the atrial or ventricular chambers of the heart. These types of approaches are generally best suited for placement within the ventricular apex or the atrial appendage. However, studies have indicated this may not be the most desirable location to provide electrical stimulation for pacing applications.
In a normal heart, a contraction results from the spontaneous excitation of the sinus node cells located in the upper right atrium. The electrical activity spreads throughout the atrium and eventually reaches the atrio-ventricular (AV) node. After passing through the AV node, the activity proceeds through nerve fibers called the His Bundle to the right and left bundle branches. From the bundle branches, the signal propagates through many parallel exits to the ventricular myocardium. The resulting depolarization wave spreads through the muscular tissue of the ventricle to cause the ventricle to contract, producing the ventricle pumping action.
As may be apparent from the above discussion, providing pacing and sensing at the apex of the right ventricle bypasses some of the body's normal conduction mechanism. That is, the conduction pathways of the His Bundle and right and left bundle branches are not utilized to sense natural heart depolarizations and to conduct a pace-triggered depolarization. As a result, conduction within the right and left ventricles may not be optimally synchronized. A more effective form of stimulation may involve pacing the His Bundle, which improves the synchronization between contractions in the left and right cardiac chambers.
Currently available endocardial screw-in leads have fixation helixes that are relatively short to avoid perforation through the heart wall when the fixation helix is rotated. For example, the MEDTRONIC® Model 5076 extendable-retractable endocardial screw-in lead, has a distal fixation helix that is about 1.8 mm in axial length. The fixation helix is coupled to the lead conductor and retractable and extendable from a distal housing through rotation of a torque tool applied to the lead connector assembly that rotates the lead conductor with respect to the lead body. The fixation helix tip is fully retracted into the lead body distal end housing during transvenous advancement through the vascular system and the tricuspid valve, when the fixation helix is to be screwed into the right ventricular wall. Due to frictional losses of torque, in many cases, it is necessary to rotate the torque tool through more turns than the fixation helix turns to ensure that the distal fixation helix is rotated through its number of turns. But, the physician cannot observe the number of turns that the fixation helix makes, and over-rotation can occur. This is one reason why the number of turns and the axial length of the fixation helix are limited.
It has been proposed to affix endocardial screw-in fixation helixes along the septal wall separating the right and left atrium as shown, for example, in U.S. Pat. No. 5,246,014 to Williams et al. However, prior art fixation helixes and other fixation mechanisms are not particularly adapted for pacing the His Bundle. This is because prior art fixation mechanisms are generally not long enough to penetrate the His Bundle and directly stimulate it to capture the cardiac tissue. What is needed, therefore, is a fixation mechanism that is adapted for fixation in proximity to the His Bundle.